package Stack;

import java.util.Arrays;

public class MyStack2<E> {

    public Object[] elem;
    public int usedSize;

    public MyStack2() {
        this.elem = new Object[10];
    }

    public void push(E val) {
        if(isFull()) {
            //扩容
            elem = Arrays.copyOf(elem,2*elem.length);
        }
        elem[usedSize] = val;
        usedSize++;
    }

    public boolean isFull() {
        return usedSize == elem.length;
    }

    public E pop() {
        if(empty()) {
            return null;
        }
        E oldVal = (E)elem[usedSize-1];
        usedSize--;
        return oldVal;
    }

    public E peek() {
        if(empty()) {
            return null;
        }
        //int oldVal = elem[usedSize-1];
        //usedSize--;
        //return oldVal;
        return (E)elem[usedSize-1];
    }

    public boolean empty() {
        return usedSize == 0;
    }
}